stdenv/cygwin: add cross bootstrap tools

[corngood]

I've built a working stdenv based on this, but I think this is a good starting point to get it merged.

Once this is in we should be able to get a tarball from hydra.

I'm marking this draft until I enumerate the package changes required for it to build.

Build with:

nix build -f pkgs/top-level/release-cross.nix bootstrapTools.x86_64-pc-cygwin.unpack

Dependencies:

• cc-wrapper: cc-wrapper: set gl_cv_clean_version_stddef=yes on cygwin #468206
• {cc,bintools}-wrapper: {cc,bintools}-wrapper: use executable extension when set #479741
• libxcrypt: libxcrypt: fix cygwin build #476257
• file: file: fix cross build for windows platforms #476283
• gnumake: gnumake: fix build on cygwin #476284
• gzip: gzip: fix cygwin build #476285
• zlib-ng: zlib-ng: fix cygwin build #476286
• gettext: gettext: fix cygwin build #476288
• gnutar: gnutar: fix cygwin build #476289
• mpfr: mpfr: fix cygwin build #476290
• gmp: gmp: fix cygwin build #476291
• isl: isl: fix cygwin build #476292
• coreutils: coreutils: fix cygwin build #476294
• git: git: fix cygwin build #476295
• deterministic-uname: deterministic-uname: add cygwin support #476296
• binutils: binutils: fix cygwin build #476298
• cygwin-dll-link: cygwin-dll-link: improvements from testing on native cygwin #476301
• gcc: gcc: fix cygwin build #476299

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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